She might be shocked at first. After all, seeing someone suddenly turn into a baby right in front of you is not an everyday occurrence. Then she could feel guilty, thinking it was her fault in some way, especially if it happened because of something related to her move. She'd probably start trying to figure out how to turn him back to normal.
At the very beginning, she would likely be extremely confused. But then, her natural nurturing side might kick in. She might start taking care of the baby - like guy, feeding him, changing his diapers etc. As time goes on, she could become more determined to find a solution to the problem. She would start looking for clues in the things they were moving, or she might recall any strange events during the move that could be related to his transformation.
One possible plot could be that the girl is secretly a witch. When the guy is helping her move, he accidentally breaks an old family heirloom of hers which has some sort of magic in it. As a result of this, she casts a spell on him in anger which turns him into a baby. Then the story could revolve around how he tries to communicate to her as a baby to reverse the spell, and she has to take care of him in the meantime, leading to some comical and also heart - warming moments as she realizes she may have overreacted.
The other characters' reactions would be diverse. Bakugo might initially be very confused and maybe even mock Midoriya a bit, but deep down he'd still see her as a rival. Uraraka would probably be very empathetic and quickly become a good friend to Midoriya in her new form, helping her with things like getting used to the new body. And All Might would be very supportive, seeing it as just another obstacle Midoriya has to overcome in her journey to become a great hero.
Some of the male characters might be a bit flustered around Naruto as a girl. For example, Kiba might act differently, not quite sure how to interact with her in this new form. Hinata, on the other hand, could become even more shy. She already had a crush on Naruto as a boy, and now with Naruto as a girl, her feelings could be even more complex. The Akatsuki members, if they were to encounter Naruto in her new form, might underestimate her at first. But Naruto, with her indomitable spirit, would soon show them that she's still a force to be reckoned with, regardless of gender. And the reactions of the characters would not only be about how they see Naruto, but also how Naruto's new form affects the power dynamics and relationships within the Naruto universe.
Her fighting style might become more fluid. Since, generally, female bodies have different centers of gravity compared to male bodies, she could adapt a more acrobatic and graceful fighting style. For example, she might incorporate more spins and flips into her attacks, using her new - found agility to her advantage.
Her character development would likely involve more self - discovery in terms of her femininity. She'd have to figure out how to be confident in a different body. Also, her relationships with male characters might become more complex, with new dynamics emerging.
Alphonse would be very protective. He'd probably be constantly worried about his baby brother's safety. He might be a bit clumsy at first with taking care of a baby, but his determination would make him learn quickly.
The guild master, Makarov, would be very protective. He'd see it as his duty to keep baby Erza safe. He might assign different members of the guild specific tasks like getting baby supplies or researching spells. Juvia would be over - the - top in her care for baby Erza. She'd constantly be fussing over her, making sure she's warm, clean and happy.
She could get a bit angry. After all, it's not a pleasant experience to have a dog mount you without warning. She might scold the dog or look around for its owner to complain.
The straight guy might initially be in a state of disbelief. He could then get angry, feeling that his boundaries have been crossed. However, he might also be confused. Maybe he has some underlying feelings that he wasn't aware of before, which could lead to a period of self - reflection.
Scott would probably be very protective. He'd feel responsible for taking care of Stiles and making sure he doesn't get into too much trouble. Lydia might be initially shocked but then use her intelligence to try and figure out how to reverse the situation. And Derek, well, he'd probably be a bit annoyed at first but would also end up being one of Stiles' protectors.